1. Field of the Invention
This invention relates to semiconductor wafer processing, and more specifically to the steering of a rail-mounted vehicle transporting a semiconductor wafer, lithographic reticles, or liquid crystal flat panel displays, during processing.
2. Description of Related Art
Automated material handling systems (AMHS) are an essential part of a semiconductor chip fabrication process, typically used as the transport mechanism for the carriers that contain semiconductor wafers. In a 300 mm fab these carriers could be a Front Opening Unified Pod (FOUP) or Reticle Storage Pod (RSP). Fast, efficient, and particle free routing of wafer FOUP or RSP transport vehicles is paramount to world-class performance of a semiconductor manufacturing process. Typically, the wafers are transported in a FOUP on top of transport vehicles that run on a track system. The alignment and placement of the FOUP or RSP is governed in part by the steering and control of the transport vehicle. A critical process in transporting FOUPs or RSPs in this system is having the vehicle make turns while the FOUP or RSP is in route. For turning, mechanical systems of the prior art currently in place rely upon a merge/diverge mechanism or lever arm to make a vehicle turn left or right on the track. Similar to a train rail, the lever arm is activated to switch from one track to another when a change in direction of the vehicle is desired. The vehicle turning is performed using such levers, bars, or rollers that affect a slow transition for turns. However, the mechanics used to make these turns put contacting surfaces under frictional stress, which generate particles that contribute foreign material (FM) contamination to a clean room environment which can be correlated to yield loss in the semiconductor manufacturing process. Current overhead transport vehicles include those taught by Daifuku, Murata, Shinko, and Brooks-PRI.
In a series of Japanese patents related to magnetic conveyors, patent no. JP08-163712 issued to Furukawa Shohei, entitled “MAGNETICALLY LEVITATED CONVEYOR,” patent no. JP04-217802 issued to Yoneda Tadao, et al., entitled “MAGNETIC LEVITATION CARRIER,” and patent no. JP04-125007 issued to Murato Masanao, et al., entitled “MAGNETICALLY LEVITATED CONVEYOR,” methods of steering semiconductor transport vehicles including stabilization and control have been achieved by electro-magnetically levitating the transport vehicle from the track. Tadao teaches using electro-magnets and guide cap sensors disposed at four points in the front and rear of a vehicle or slider that levitates the vehicle magnetically along guide rails. Shohei teaches mounting a levitation electro-magnet at the truck to magnetically attract a rail at the linear part of a conveying passage. A guide electro-magnet provided on the truck magnetically attracts guide rails provided along both sides of a curved portion of the track.
All of these inventions provide electromagnetic levitation to lift the transport vehicle. However, in each instance the vehicle is levitated and transported on a center rail, which acts as the directional guide for the vehicle. Electro-magnets have not been used in the prior art to initiate and negotiate turns. These series of patents, however, do not teach using electro-magnets for steering and controlling, rather than levitation, or suggest suspending and driving the transport vehicle with an air cushion, or having the transport vehicle supported by wheels and controlled by electro-magnets for selecting the proper vehicle path.
Due to inertial forces and magnetic levitation, electromagnets can be used in various configurations to control the uniform travel of a transport vehicle within a curve or straight section of track. In order for the vehicle to ride uniformly in a cross-direction or tilt in the curves and straight sections, various combinations of electromagnets must be deployed.
FIG. 1A depicts a prior art transport vehicle 100 that travels on wheels 102 and is guided down the track by guide wheels or rollers 101. This design is similar to the one taught by Murata. FIG. 1B depicts the undercarriage of the transport vehicle 100 of FIG. 1A. The vehicle is propelled by a linear magnet stepper motor 140 and a magnet propulsion motor 106, to guide it down a track 139. FIG. 1C details the physical relationship between the main support wheels 102, the guide wheels or rollers 101, and the linear magnet propulsion motor 106.
The prior art uses a merge/diverge system, as depicted in FIGS. 2 and 3, to steer and direct the transport vehicle 100. For illustrative purposes, the straight section of track 103 shown in FIG. 2 leads into another straight section 106. A left turn track portion 107 is attached at the junction of straight sections 103 and 106. When the computer controller of the track transport system determines that vehicle 100 should be going in a straight direction on section 103 to section 106, the merge/diverge lever 104 is moved to position 104. If the vehicle is required to turn to the left on track section 107, the merge/diverge lever 104 is moved to position 105, as shown in FIG. 3. This is a slow transition from one designated position to another, requiring movement of discrete parts with potential for mechanical failure. Moreover, the motion and frictional wear generates airborne contaminating particles. Additionally, when the vehicle guide rollers/wheels 101 traverse a merge/diverge lever, the vehicle experiences a significant amount of vibration.
The known solutions degrade the performance of existing vehicles because the vehicles must slow down when approaching and entering a turn. The turning action forces the steering rollers to ride along a guide rail switch lever, which in turn creates friction and results in particle contamination. The movement on the prior art control rails, similar to a train track switch, is slow and prone to mechanical failure.